This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Superconductors are materials in which electrical current flows with a resistance of zero, typically below specific temperatures. In conventional superconductors, this state of matter emerges when two electrons bind together at low temperatures, forming so-called Cooper pairs.
Yet some materials, referred to as unconventional superconductors, exhibit superconductivity under unusual conditions and cannot be explained by conventional theories. Understanding these unusual cases could lead to the development of superconductors that can operate at high enough temperatures to be used in real-world devices with less refrigeration. Researchers at Harvard University and the University of Stuttgart theoretically demonstrated that an unusual form of graphene, known as valley-imbalanced rhombohedral tetralayer graphene, could host unconventional superconducting states.
Their paper, published in Physical Review Letters, suggests that superconductivity in this material could simultaneously condense at multiple incommensurate momenta, leading to the spontaneous formation of a superlattice of Cooper pairs. "Our work was inspired by an experiment that showed that superconductivity can emerge out of a normal state in a specific stack of graphene layers called rhombohedral graphene, where electrons in two valleys of graphene spontaneously choose to fill one valley over the other," Mathias Scheurer, corresponding author of the paper, told Phys.org. "This is very exciting since the two valleys of graphene are related by time-reversal symmetry and thus an imbalance between the two spontaneously breaks this symmetry.
Yet this symmetry is central to superconductivity; so much so that virtually all theoretical works focusing on conventional or unconventional superconductivity begin by assuming it is not broken. As such, it was of fundamental interest to identify what the key modifications are to the theory of superconductivity if this symmetry is absent." Scheurer and his co-authors Maine Christos and Pietro M. Bonetti developed a theoretical framework that could be used to classify possible superconducting pairing instabilities in rhombohedral tetralayer graphene.
In conventional superconductors, paired electrons typically have opposite momenta (i.e., motions), so their collective net momentum vanishes. The team's framework also considers electron pairs with nonzero momentum. It also distinguishes between commensurate states and incommensurate states.
The first are superconducting patterns compatible with the periodicity of a material's underlying crystal lattice, while the latter have a periodicity that does not match the crystal lattice. "Some of us had previously worked on transport signatures of superconductors with valley imbalance, motivated by the fact that it stabilizes nonreciprocal critical currents, i.e., critical currents that differ in forward and reverse directions," said Scheurer. "As such, we had already developed a good understanding of some of the basics of the interplay of valley imbalance and pairing.
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